For example, an imaging device (image sensor) used in an imaging device has a configuration in which a color filter permitting a specific wavelength component of light (R, G, B) to penetrate a pixel unit is attached to the surface of the device. For example, a filter having an RGB arrangement, as shown in FIG. 1(a), is used. In a process of generating a color image using an output signal of the imaging device, the process of restoring a necessary color component using a set of a plurality of pixels. The color arrangement of the color filter comes in many types, and a Bayer arrangement, configured by 3 kinds of filters, which permit only a specific wavelength light, red (R), green (G), and blue (B) shown in FIG. 1(a), to penetrate, is frequently used.
In recent years, the pixels of the imaging device (image sensor) have become increasingly smaller in size, and accordingly a problem has occurred in that the amount of light incident to each pixel has decreased, and thus the S/N ratio has deteriorated. To solve this problem, as shown in FIG. 1(b), in addition to a filter permitting, for example, only a specific wavelength light of RGB, or the like to penetrate, an image sensor (imaging device) having a white filter (W: White) which additionally permits light in a visible light range to penetrate widely, has been proposed. In FIG. 1(b), one example of a filter having an RGBW arrangement is illustrated. A W pixel in the RGBW arrangement shown in FIG. 1(b) is a filter which permits light in a visible light range to penetrate widely.
In this way, an imaging device equipped with a color filter having white (W: White) pixels is disclosed in, for example, PTL 1 (U.S Unexamined Patent Application Publication No. 2007/24879) and PTL 2 (U.S Unexamined Patent Application Publication No. 2007/0024934).
As shown in FIG. 1(b), an imaging device (image sensor) having the color filter having the white (W: White) pixel is used, and thus the penetration light rate of the filter is increased and high sensitivity may be accomplished.
However, the problems with an RGBW type device are as follows.
Both the RGB arrangement shown in FIG. 1(a) and the RGBW arrangement as shown in FIG. 1(b) are single elements in which a filter of R, G, B or W is arranged in a mosaic state, that is, a single plate type image sensor. Therefore, when generating a color image, it is necessary to perform a de-mosaicing process as a color coding that generates an RGB pixel value corresponding to each pixel.
In the RGBW arrangement shown in FIG. 1(b), the sampling rates of R, G, and B components are decreased, compared with the RGB arrangement shown in FIG. 1(a). As a result, in a case where obtainment data obtained by an element with the RGBW arrangement shown in FIG. 1(b) is used when performing a process of generating the color image, there is a problem in that false color may easily occur, compared with the RGB arrangement shown in FIG. 1(a). Furthermore, because a wavelength component of the white (W) includes all of the wavelength components of R, G and B, there is a problem in that the concentration rate of an RGB wavelength component is decreased and resolution is decreased compared with that of a single color component when an optical lens, having a large amount of chromatic aberration, is used. As the pixels are smaller in size, this problem is more remarkable.
As a technique to prevent a deterioration in resolution resulting from chromatic aberration in the optical lens, it is effective to suppress an occurrence of chromatic aberration by combining lenses which are different in refractive index, but in this case, there is another problem in that costs are increased because an increase in the number of optical lenses. Furthermore, in this configuration, a problem also occurs in that the above-described problem of false color resulting from a decrease in the sampling rate of the RGB component is more remarkable.
Furthermore, because each pixel of a single plate type image sensor only has information for a single color component, a de-mosaicing process of obtaining the RGB pixel values corresponding to all of the pixels is performed to obtain the color image from the R, G, B, and W signals which are discretely obtained. When performing the de-mosaic process, an interpolation process is performed based on the assumption that a color rate is almost uniformly retained and there is a strong color correlation in the local region. Specifically, when calculating the pixel value of a specific color of a pixel, a method of interpolating using the pixel values of the neighboring pixels is widely used. This method is disclosed in, for example, PTL 3 (Japanese Unexamined Patent Application Publication No. 2009-17544). However, the above-described assumption that the color rate is almost uniformly retained and color correlation exists in the local region is not true for the neighborhood of the edge. As a result, there is a problem in that the false color easily occurs in the neighborhood of the edge.